Design and Development of Strain Temperature Testing Machine

The success and outcome of this project required a lot of guidance and assistance from many people and we were extremely privileged to have got this all along our project journey. All that we have done is only due to such supervision and assistance. It is our radiant sentiment to place on record our best regards, deepest sense of gratitude to our advisor Dr. Muhammad Imran Khan for providing us an opportunity to do the project work as our final year project and giving us all the support and guidance throughout the project. We heartily thank our co- advisor Dr. Zia –Ul-Haq Abbas, who took keen interest in our project and was a constant source of encouragement, support and guidance throughout the project. Also, we would like to extend our sincere esteem to all the staff and laboratory assistants in the faculty of Materials science and Engineering and faculty of Electrical engineering for their timely support.

Abstract

Elevated temperature tensile testing is the method of predicting a materials behavior under tension and excessive temperature applications. The process is similar to the conventional tensile testing method with a slight modification. The test sample instead of being tested at room temperature is tested at higher temperature ranges using a furnace (W. F. Hosford, 1992). The furnace is enclosed around the test sample and the temperature is elevated at a constant rate as the predefined load is applied on the sample. This project aims to design and develop a strain temperature testing machine in order to study the relation between the varying temperature at a constant rate and the strain induced in the sample at a predefined stress applied. The cross beams were made of stainless steel to support the load applied. Two pull rods to grip the specimen were also fabricated from stainless steel, capable of withstanding the maximum heating range of the furnace which is 400 degree Celsius. The furnace is designed such that it is able to enclose around the test specimen with cavities through both top and bottom for the pull rods to pass through and grip the sample inside the heating zone of the furnace. Furnace is fabricated from the refractory material and tungsten coil is used as a heating element. The coil is controlled by a temperature control unit and a thermocouple sensor is used to sense the temperature changes at heating zone of the furnace. Instead of applying the load through the hydraulic or electromechanical mechanism, weights were used which assisted in reducing the complexity of fabrication and was cost effective. A weight stand was welded to the bottom pull rod and the weights were placed in the stand in order to apply the required load. In order to measure strain induced in the sample as the result of applied load an extensometer is used.

Literature review

There are several reasons to perform tensile testing. The results of these tests are used in order to select an appropriate material for a specific engineering application. To insure the quality the tensile properties are frequently included in material specifications. During the development of new materials and process, the tensile properties are to be measured in order to compare different materials and processes (Mumford, 1992). Most importantly the tensile properties of a specific material are used to study its behavior under different forms of loading. Mostly the strength of a given material is of utmost concern. The strength of a material under consideration can be measured in two ways; either the maximum amount of stress that a material can withstand before it fractures or the minimum amount of stress required to deform the material plastically. The ductility of a material is also a major concern, it measures the amount of deformation a material can withstand before it fractures. Ductility measurements are not directly included in design rather, to ensure the toughness and quality it is incorporated in material specifications. If the ductility of a given material is low, it corresponds to low resistance to fracture under loading. All these measures of strength are used in engineering design while taking the safety factors into the account. Special methods such as ultrasonic techniques maybe used to achieve more accurate measurements (Gedney, February, 2002).

Testing machines

The universal testers are the most common type of testing machines, they are used to test the material under compression, tension or bending. The main function of the test is to generate the stress strain curve, which is then interpreted to study different behavior of a material. There are two types of testing machine variants, hydraulic or the electromechanical. Main difference between the two is the load application mechanism (Harvey, p. 1968). In the hydraulic testing machines the crosshead movement is controlled by either a single or dual acting piston. Single acting piston is more common in static hydraulic testing machines. To control the rate of loading in manual hydraulic machines, the operator has to adjust the orifice of the needle valve. While in the case of closed loop hydraulic servo system, an electric servo valve is used instead of the needle valve resulting in more precise control (Zhu, 2011). On the other hand in Electromechanical testing machines include an electric motor and the gear reduction system which is capable of moving the crosshead up or down with the help of multiple screws. The specimen can be loaded in tension or compression as a result of crosshead movement. By altering the speed of the motor one can control the crosshead movement. In order to control the crosshead speed more accurately, a microprocessor based servo system can also be installed (Davis, 2004).

Elevated Temperature Tensile Testing

High temperature tensile testing is a method to determine the behavior of a certain material when it is subjected to tension and excessive heat. In order to assess high performance steels and other such metals which are to be used at elevated temperature conditions, these high temperature tensile tests are performed on a regular basis, other materials include nickel alloys often used for jet engines and gas turbines (Hart, 1967).

Sample Selection

The main objective to test any material is to know if the given material is suitable for a specific use. It should be made sure that the test specimen represents the material under observation. The test specimen should have gone through the same processing procedures and have the same source as material. Most of the times it is quite difficult to exactly replicate the test samples to the structure of the material under consideration. Most commonly extra material is added to the part to be used as “built in” samples in the testing of large castings. The extra material is then separated from the part and turned into test samples to achieve higher accuracy. Certain major details of these test specimen may not completely replicate the bulk material for example the grain patterns of the forging critical areas. (Sergueeva, 2009) To compare the test samples of the critical areas with the built in samples multiple number of complete parts may be used. This is one way to determine how well the built in samples represents the material under observation. In some cases the main objective is to check the quality of the test rather than the material itself. To ensure that the differences in the test results only account for the variance in the testing procedure, the test sample should be as identical as possible.

Applications of high temperature Tensile Testing

Tensile testing ensures that the right material is being chosen for a specific job within a wide range of industries. Some of the major industries where tensile testing is being used are as follows

• Aerospace Industry
• Automotive Industry
• Beverages Industry
• Construction and Building Industry
• Electronics Industry
• Medical Equipment Industry
• Packaging Industry
• Paper Industry
• Pharmaceuticals Industry
• Plastics and Elastomers Industry
• Health, safety and Fitness Industry
• Textiles Industry

Results and conclusion

After going through all the previously mentioned design and fabrication steps we were able to make a basic working model of the elevated temperature tensile testing machine to be used at the laboratory level. The furnace was successfully tested, we were able to increase the temperature from room temperature to 400C at a constant rate. The thermocouple sensor was tested to sense the temperature variations at the heating zone of the furnace. Choosing conventional weights to be hanged from the lower pull rod in order to apply the required load proved to be very cost effective.

Moreover it reduced the complexity of design and fabrication of the machine. We were not able to carry out a complete elevated temperature tensile test of a sample. We could not procure the grips required to hold the test specimen at elevated temperature and the extensometer to measure the deformation of the sample. Nonetheless with some modifications and further improvements, this model could be used to test samples at elevated temperatures for research purposes at the laboratory scale as a cost efficient alternative to the standard high temperature testing machines available in the market.